In recent years, it has become more and more common to use cellular radio communication systems for automatic exchange of information between devices for performing many different tasks, such as opening and closing of valves in for example a sewage system, measuring of temperature or pressure, updating of map information for a Global Positioning System (GPS-system) in a car and more. This kind of automated communication without user interaction is often referred to as machine-to-machine (M2M) communication. Typically, these autonomous devices transmit or receive only small amounts of data, more or less infrequently, such as one per week to once per minute. According to 3rd Generation Partnership Project (3GPP) standardization terminology, these devices are often referred to as machine type communication devices (MTC devices). Sometimes MTC devices are denoted machine devices (MDs) in the literature. In the context of this document MTC devices are able to communicate with and via a cellular network. As such, an MTC device may also be referred to as a mobile terminal, or, using a well established 3GPP term, user equipment (UE). An MTC device may thus be seen as a special type of UE and in this document MTC devices will sometimes be referred to as MTC devices and sometimes as UEs. As more and more devices, such as laptops, digital cameras, cars, outdoor thermometers, indoor thermometers, electricity meters and so on, become connected, the number of connections in the radio communication systems will increase drastically.
With the nature of MTC devices and their assumed typical uses follow that they will often have to be very power efficient, since external power supplies will often not be available and since it is neither practically nor economically feasible to frequently replace or recharge their batteries.
One possible means to achieve low energy consumption in MTC devices is to use long active/connected mode Discontinuous Reception (DRX) cycles with long inactive/sleep periods. The Discontinuous Reception (DRX) procedure, defined as a part of the LTE Medium Access Control (MAC), specifies time periods during which a communication device is obliged to monitor the Physical Downlink Control Channel (PDCCH). In DRX, an active time is defined for this purpose. In time periods, specified as active time, the communication device is not allowed to go to a sleep state. For simplicity, time other than active time is referred to as sleep time even though the user equipment (UE) is not required to go to the sleep state. However, since the UE consumes less power in the sleep state than in the active time, it is beneficial to enter the sleep state. Active time is calculated based on specific DRX timers and cycles in such a way that the network and the communication device have a similar understanding of when it is possible to schedule the communication device.
When the UE remains in connected mode DRX with long (i.e. longer than typically used for regular UEs) DRX cycles, a long active/connected mode DRX is said to be used. The evolved radio network node of an LTE network (eNB) maintains a context (i.e. state information) for a UE in long active/connected mode DRX, even during the sleep periods. In contrast, a UE in idle mode DRX is in idle mode during both sleep periods and active/listening periods and the eNB thus has no context for the UE.
Therefore, when using the long active/connected mode DRX procedure the MTC devices is allowed to remain in connected mode and still spend most of their time in an energy-efficient sleep mode. An advantage that connected mode DRX has over the idle mode DRX is that the MTC device does not have to go through idle-to-connected mode transition before transmitting or receiving data. This saves signaling overhead and thus both radio resources and battery.
In 3GPP Long Term Evolution (LTE), all scheduling assignments, grants and commands are issued to specific Radio Network Temporary Identifiers (RNTI) on the Physical Downlink Control Channel (PDCCH). The RNTI is a number between 0 and 216−1. Different types of RNTIs exist, such as the Paging RNTI (P-RNTI), System Information RNTI (SI-RNTI), etc. For example, a communication device (or user equipment, “UE”) that is reading e.g. System Information is looking for the commands assigned to the SI-RNTI on the PDCCH. RNTIs can either be common to several communication devices, or unique to one specific communication device. Scheduling assignments and grants are sent to the user equipment by means of downlink control information (DCI) messages which are encoded with a RNTI, such as C-RNTI, on the PDCCH.
Specifically, the Cell RNTI (C-RNTI) is used to address a specific communication device in a connected state, such as RRC_CONNECTED state in case of an LTE system. A communication device in RRC_CONNECTED state has established a connection to a cellular radio communication network. Therefore, the communication device in RRC_CONNECTED state needs at least one C-RNTI that is unique among the C-RNTIs assigned to other communication devices in RRC_CONNECTED state in the same cell. Multiple RNTIs may be allocated to a communication device at the same time, i.e. in parallel. For example, a Semi-Persistent Scheduling RNTI (SPS-RNTI) may be assigned to a communication device in addition to the aforementioned C-RNTI.
However, the C-RNTI is not explicitly included in the Downlink Control Information (DCI). Instead, it is implicitly included in a Cyclic Redundancy Check (CRC) value attached to a DCI payload. First the CRC is calculated and then the RNTI is added (bitwise modulo 2) to the CRC before the block is coded, modulated and transmitted. The receiver may use the following procedure: Calculate a CRC on the received payload and then subtract it from the received, modified CRC. The result of the subtraction is, provided that no bit errors have slipped through, the encoded RNTI, which may then be compared with any applicable RNTI in search of a match.
Moreover, PDCCH signaling may be transmitted on several sets of resources using different formats and schemes for coding and rate matching. A resource is defined by a range in time and frequency in a time-frequency grid of LTE.
This forces a UE/MTC device monitoring the PDCCH to blindly decode multiple different PDCCH formats on multiple different sets of resources in search of correctly decoded message addressed to one of the RNTIs (e.g. the C-RNTI) allocated to the UE/MTC device. To limit the complexity of the UEs/MTC devices the sets of resources that the eNB may transmit the PDCCH signaling on is restricted by certain rules. First, resource elements that may be used for PDCCH signaling are grouped into so-called Control Channel Elements (CCEs). Secondly, only certain combinations of CCEs are allowed. To further limit the number of resource element combinations that a UE/MTC device has to search for relevant PDCCH signaling, UE/MTC device specific, further restricted combinations of CCEs are introduced. Such a UE/MTC device specific combination of CCEs is referred to as a search space. A UE/MTC device specific search space is derived from the C-RNTI in combination with the subframe number. In addition to the UE/MTC device specific search spaces there are common search spaces, which are searched by all UEs/MTC devices and which are used for signaling with multiple receivers, e.g. scheduling assignment for system information transmissions. UE/MTC device specific search spaces may often overlap each other, which means that a UE/MTC device may sometimes be blocked from PDCCH signaling in a subframe, if the CCEs in its search space are already used for PDCCH signaling to other UEs/MTC devices with overlapping search spaces.
The RNTIs in current LTE network are signaled by 16 bits, meaning that 216=65 536 values are possible. However, in practice, it can be speculated that if allocations of RNTIs are very closely in the RNTI space (in terms of the Hamming distance), this would lead to a high probability of RNTI misdetection. If this is the case, it is possible that only a fraction of the current RNTI number space can be utilized in practice.
The following problems make the C-RNTI values limited:
                all Radio Resource Control (RRC) connections need at least one RNTI, i.e. the C-RNTI,        only one connection can be identified with one C-RNTI, and        some connections may require multiple RNTIs.        
Furthermore, as explained above, not all RNTI values are available for C-RNTI use, but only a subset (albeit a large one) is actually allocated for C-RNTIs.
Consider the following scenario. It is assumed that data becomes available for transmission in the communication device, but the communication device does not have uplink (UL) resources to transmit the data, even when the communication device is in the RRC_CONNECTED state. Thus, the communication device sends a Scheduling Request (SR) to requests resources from a radio network node, such as an eNB, provided that the communication device has been allocated Physical Uplink Control Channel (PUCCH) resources for transmission of the SR. If no PUCCH resources for SR transmissions are assigned to the communication device, the communication device initiates a Random Access (RA) procedure. In a contention based RA, the communication device selects a random preamble to be transmitted on a Random Access Channel (RACH). For this case, the RA procedure is as follow:                The communication device transmits a random preamble selected by it on RACH (as noted above).        The radio network node responds with a RA Response (RAR) for the same preamble as transmitted by the communication device. RAR message includes a Scheduling Grant (SG) for an UL transmission, also known as “UL grant” or “Random Access Response Grant”.        The communication device now responds to the RAR with a scheduled message 3 (as known from 3GPP-terminology) including a C-RNTI thus identifying the communication device.        The radio network node then replies with a Contention Resolution message. If the Contention Resolution message includes the same C-RNTI as the communication device has transmitted in message 3, the communication device regards the Random Access Procedure as successful.        
As mentioned above, the RA Response contains an UL grant. In conjunction with the RA procedure the radio network node also has the possibility to allocate uplink signaling resources on the PUCCH, which the communication devices may use to request further uplink transmission resources from the radio network node. The communication device may also send a scheduling request together with any uplink transmission on the PUSCH, e.g. the uplink transmission using the resources assigned by the UL grant in the RA Response.
The above described method is contention based, because two communication devices can request resources at the same time with the same preamble. In this case the radio network indicates by means of C-RNTI in Contention Resolution message which of the communication device succeeds with the random access.
As the number of connections, due to for example automated communication from communication devices in the radio networks increases, it is possible that the current number of usable RNTI values is not enough to cater for all the devices in a cell simultaneously. An example where this may happen is a dense sensor network including a huge amount of temperature/pressure/humidity sensors. In addition, there may be other user equipments, such as cellular phones, in the same cell as the sensors. These user equipments may also be connected and, hence, consume (or occupy) a C-RNTI each. Thus, each communication device requires a C-RNTI that is unique in the cell.
When the RNTI space is exhausted, the network needs to drop connections of some devices to allow for other devices to connect instead. Switching frequently between connected state and idle state increases the amount of signaling messages, overhead and also battery consumption. For small devices having only very limited battery, this is not desirable.
Looking primarily from a 3GPP Evolved Packet System (EPS) perspective, there are problems associated with the existing technology. Although staying in idle mode during inactivity and using long paging DRX cycles is efficient, it requires the MTC device to go through the RRC connection (and EPS bearer) establishment procedure, i.e. the idle-to-connected mode transition, every time it needs to transmit or receive some data. This causes a lot of signaling overhead and also consumes energy in the MTC device.
The connected mode DRX eliminates the need for idle-to-connected mode transition, but it has other disadvantages. One disadvantage is that in mass deployment scenarios a cell may run out of C-RNTIs, because of the potentially large number of devices that are kept in connected mode simultaneously. This is due to the limited 16-bit length of the C-RNTI which is sufficient for traditional UEs but may be too limited for massive amounts of machine devices.